The present invention relates generally to tactile sensors. More particularly, the invention describes capacitive tactile sensors which combine elements of traditional touch sensors and traditional tactile sensor in a novel way.
Traditional capacitive tactile sensors feature arrays of electrodes having two layers of electrodes (typically columns and rows) that need to be connected to control electronics. The layers are separated by a compressible dielectric layer. Control electronics for measuring capacitance may be incorporated into ASICs, which makes it much easier to develop smaller compact sensor electronic solutions.
Capacitive tactile sensors are used to measure both the location of touch as well as touch force or pressure distribution at that location. There are two types of capacitive tactile sensors. A discrete element capacitive tactile sensor typically has one element for one input channel on the capacitance-sensing integrated circuit (IC). A multiplexed array capacitive tactile sensor on the other hand uses an array of a plurality of electrode rows and separate electrode columns to maximize the number of sensing elements while minimizing the number of interconnections to the capacitance-sensing IC.
In essence, mechanical deformation of the compressible layer is detected by changing capacitance between adjacent electrode layers and translated by the control electronics into location and force of touch. In that sense, capacitive tactile sensors respond to a mechanical deformation from an external object.
While fabricating flexible tactile sensors, reliability of interconnections becomes a weak point in the design since the sensor electrodes are often fabricated using conductive materials that are different than copper-clad polyimide flexible printed circuit boards (FPCB). Even when the same FPCB is used, the interconnection between the two layers of electrodes and the control electronics is a weak point for sensor reliability. For the purposes of this description, the term “flexible printed circuit board” or FPCB is used to broadly depict a flexible polymer dielectric substrate on which a plurality of electrodes is placed using any known deposition manufacturing methods. Examples of such methods include printing, photolithographing techniques, laminating electrodes in between polymer films, bonding, and other methods known in the art. Included in the definition of the term “flexible printed circuit board” are printed electronics made with common printing equipment suitable for defining patterns on substrate material, such as screen printing, flexography, gravure, offset lithography, and inkjet. The term “flexible printed circuit board” is used to depict one-sided or two-sided designs of flexible circuits with electrodes placed correspondingly on one or both sides of the flexible polymer substrate. Further included in the term “flexible printed circuit board” are double access flex circuits, back bared flex circuits, sculptured flex circuits, multi-layer flex circuits, roll-able electronics, stretchable electronics, organic electronics, plastic electronics, rigid-flex circuit hybrids, flex circuits with one of more metals used as conductors and other known designs in the art of flexible circuits.
The array of electrodes for the capacitive tactile sensor employs a plurality of individual column 10 and row 20 electrodes as seen in FIG. 1. Column 10 electrodes are typically connected to the control electronics using individual interconnections 11 made as electrical wires, while row 20 electrodes are connected to same using individual interconnection wires 21. FIG. 2 shows another prior art capacitive sensor array in which electrodes of rows 20 are directly attached to a FPCB using interconnections 22 while electrodes of columns 10 use jumper wires 12 to connect to the FPCB.
Typically, the width of the electrode strip in these capacitive tactile sensors is selected to be close to the gap between adjacent fine conductors. That is done to maximize the capacitance between the drive and the sense electrode.
A shield layer may also be used in order to protect the sensor from electrical interference since the purpose of a tactile sensor is to measure the mechanical contact pressure distribution.
Capacitive touch sensors are different from the capacitive tactile sensors in that they do not contain any mechanically deformable layers. These sensors work on the principle of detecting an attenuation of the coupling field formed between the drive and sense electrode layers when a conductive object is present in their vicinity, such as a human finger. In this sense, capacitive touch sensors operate as electric activity sensor—they are not responsive to mechanical deformation of any of their layers. An example of a prior art capacitive touch sensor can be found in U.S. Pat. No. 8,599,150 incorporated herein in its entirety by reference.
Capacitive touch sensors are not deformable and because of that they can only be used to detect the location of touch and not the force of touch or pressure distribution. Capacitive touch sensors do not have a compressible dielectric layer inbetween drive and sense layers of electrodes so the electrodes for both drive and sense sides may be applied to a two-sided FPCB. This greatly improves reliability of interconnections with the control electronics and simplifies production of such sensor.
Electric patterns for touch sensor electrodes vary widely—between a “checkered board” pattern to a multiple strip electrodes with large gaps between adjacent conductors. Capacitive touch sensors use different electrode patterns from capacitive tactile sensors, which employ wide strips as array electrodes. Another difference is that a capacitive touch sensor does not employ a distinct conductive shield since the human finger or another grounded or conductive object is the shield in itself that disrupts the electrical coupling fields between the drive and the sense electrodes. This is the reason why a capacitive touch sensor does not work when a user is wearing a non-conductive glove. This prompted glove manufacturers to develop gloves with conductive finger tips to allow operation of a traditional capacitive touch sensor.
The need therefore exists for a capacitive tactile sensor capable of measuring both the location of touch as well as a force of touch with high resolution and improved reliability of interconnections between the array and the control electronics, especially when using flexible surfaces.